Aha! The Bible describes the Cross as foolish!
The passage under question today is 1 Corinthians 1:18-25
18For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God. 19For it is written,
"I will destroy the wisdom of the wise,
and the discernment of the discerning I will thwart."
20 Where is the one who is wise? Where is the scribe? Where is the debater of this age? Has not God made foolish the wisdom of the world? 21For since, in the wisdom of God, the world did not know God through wisdom, it pleased God through the folly of what we preach to save those who believe. 22For Jews demand signs and Greeks seek wisdom, 23but we preach Christ crucified, a stumbling block to Jews and folly to Gentiles, 24but to those who are called, both Jews and Greeks, Christ the power of God and the wisdom of God. 25For the foolishness of God is wiser than men, and the weakness of God is stronger than men.On face value it seems quite obvious that the author of 1 Corinthians (Paul) is admitting that Christianity is a foolish and consequently quite irrational belief. This would be odd coming from a man who was convinced of Christianity's truth and who spent time debating his belief with philosophers and people of Jewish faith. What key can unlock this passage?
Wisdom is the key!
You'll notice that wisdom is a prominent theme throughout this passage. But what does it actually mean? Well in the Bible there is a whole book dedicated to wisdom: Proverbs. If you read Proverbs you'll quickly notice that it is concerned with moral matters and how to make practical decisions. It isn't about 'facts' or 'knowledge' as such, more how we actually apply our knowledge to real life. Wisdom, Biblically speaking, has a lot to do with morality (J.P. Holding, our source for other contextual concerns, concurs). So how does this understanding of wisdom help us to understand this passage?
We should recognise that the central Christian claim - that Jesus was God in the flesh, that he was crucified, and that he was resurrected in a body - was very much against the socially ingrained way of thinking about God and what was honourable. Christianity would have been offensive and would have seemed utterly foolish to the people of the time. It was very much against conventional wisdom. J.P. Holding outlines a number of factors that made Christianity appear foolish over here, but we shall only need to look at one.
A dishonourable death
The two people groups mentioned in 1 Corinthians 1:18-25 are Jews and Gentiles (non-Jews, specifically Greeks). Both at this time and place in history were hugely concerned with honour. What was honourable always dictated what was morally right to do. The trouble is, crucifixion was an utterly dishonourable death. A religion centred around this sort of execution would seem literally foolish, especially when the person crucified is said to be God - the very being deserving of the highest honour! As such the central Christian claim was contrary to the wisdom the Greeks sought as God's valuation of what is honourable differed from theirs. As for the Jews seekings signs, they were after miraculous wonders that would overthrow their Roman occupiers and establish Israel as God's kingdom on earth. They weren't after a crucified messiah; God's estimation of what was the wise thing to do differed from theirs.
In conclusion this passage does not teach that Christian ought to be ignorant or actually foolish. Only that Christians should expect that people will consider their beliefs foolish given how contrary they can be to accepted wisdom.